1. Technical Field
The present invention relates to measuring techniques, in particular, to optical pyrometry of molten metals and, more specifically, to a light guide for transmitting thermal radiation from a melt to a pyrometer, and also to a method of measuring temperature of a melt in a metallurgical vessel with the use of said light guide.
2. Description of the Prior Art
Alongside with the unquestionable advantages the optical pyrometry has over other methods used for measuring temperature, in particular, over the measuring method using thermocouples immersed in a melt (a possibility to provide continuous temperature control, non-contact measuring process, etc.), the use of optical pyrometry for industrial production purposes is associated with certain difficulties. In particular, pyrometer readings depend to a large extent on optical characteristics of the radiating surface and of the intermediate medium.
One of the most promising fields in optical pyrometry of molten metals is the development of methods based on the use of light guides forming an isolated channel for transmitting thermal radiation from a melt to a pyrometer thus allowing the influence of the abovementioned factors on measurement accuracy to be decreased.
An important problem which specialists in this field of optical pyrometry are faced with is the development of light guides highly reliable in operation, having satisfactory optical characteristics and providing more accurate temperature measurements even in the case of using pyrometers which are simple in construction.
A great number of patents have been granted recently in various countries, which shows that this is an urgent problem and that attempts are made to solve it (Cf., for example, the USSR Authors' Certificates No. 146,533, 1961, No. 271,067, 1970, U.S. Pat. No. 3,745,834, 1973, FRG Pat. No. 2,338,532, 1976). In spite of the many attempts which have been made, the above problem has not been satisfactorily solved yet.
In all known constructions the light guide is arranged, for example, in a cylinder and has a constant cross-section. In particular, such is the embodiment of the light guide for transmitting/thermal radiation from a molten metal to a pyrometer disclosed in the Austrian Pat. No. 280,650, 1970. This light guide is made from a light permeable refractory corrosion-resistant material, for example, from quartz glass.
The temperature of the melt is measured with the aid of said light guide in the following manner. The light guide is mounted in the lining of an apparatus, preferably of a metallurgical vessel filled with a molten metal, so that the operating end of the light guide is in contact with the melt. A spectral ratio pyrometer is positioned at the opposite end of the light guide. Thermal radiation caused by a high temperature of a controlled medium (melt) is transmitted along the light guide through the metallurgical vessel lining to a pyrometer by whose readings a temperature of the melt is determined.
When measuring temperature of a melt with the aid of the above light guide, the following difficulties arise. First of all it should be noted that the influence of variation of radiation ability of the light guide operating end on the measurement accuracy is not excluded. This variation may be caused, for example, by a change in the chemical composition of the controlled medium (change of a melt) or deterioration of the operating end surface during its servicing (an increase of its roughness, appearance of microcracks, etc.). In such cases one-valued correspondence of the thermal radiation of the operating end to the temperature of the latter and consequently the temperature of the melt is disturbed.
In addition, if the melt being controlled possesses such optical characteristics that radiation ability of the light guide operating end and, consequently, the intensity of the radiation being transmitted are inconsiderable, there arises a necessity to use pyrometers with highly sensitive radiation receivers.
It should also be emphasized that to decrease the influence of the variation in the radiation ability of the light guide operating end on the measurement accuracy, as has been stated above, complex and costly pyrometers are required, such as a spectral ratio pyrometer and a pyrometer with automatic correction. Nevertheless, even with the aid of such pyrometers it is not possible to completely rule out the influence of said factor on measurement accuracy.
It is also to be noted that in the prior art method of measuring temperature of a molten metal with the aid of said light guide the problem of selecting a best suited place for mounting the light guide in the metallurgical vessel lining has not been solved. When the light guide is mounted arbitrary in the lining, its operating end is frequently deteriorated due to thermal shocks (sharp temperature change) occurring when a molten metal is poured into the vessel crucible or poured out, when the furnace is charged or the vessel is inclined as well as in other cases.
In addition to this, the light guide in many cases is deteriorated in that part thereof which is set deep inside the lining, either due to the variation of the temperature gradient with thickness of the lining in the place where the light guide is mounted or due to the lining displacement with respect to the furnace shell and relative offsetting of said lining layers during operation.
It should be stressed that in so far as in the metallurgical vessel there are zones wherein the temperature of the melt considerably varies from said vessel mass-average temperature, an arbitrary mounting of the light guide will not ensure reliable information on its mass-average temperature. Let us also note that if the light guide is found in the crucible slagging zone, then as a result of change in the temperature drop between the melt and the operating end of the light guide, measurement errors will be even greater.
Thus, an arbitrary mounting of the light guide does not provide a required accuracy of measuring mass-average temperature of the melt in the metallurgical vessel, which frequently does not meet the demands of the melting process.